Method of continuously casting steel billets

ABSTRACT

In continuously casting steel billets wherein the mold is oscillated up and down substantially according to a sine function the mean speed of descent of the mold is selected so that the maximum speed of withdrawal of the casting from the descending mold during not less than about 55 percent, and the minimum speed during not more than about 80 percent, of the time required for the complete descent of the mold is below the speed of the mold. Further, the level of liquid steel in the mold is preferably kept constant by adjusting the withdrawal speed of the casting in accordance with the rate of delivery of liquid steel into the mold; if the withdrawal speed of the casting exceeds the desired maximum or falls below the desired minimum, further delivery of liquid steel to the mold is stopped.

ite tates atent Knell 5] Oct. 24, 1972 [54] METHOD OF CONTHNUOUSLY CASTING STEEL BILLETS [72] Inventor: Bernard Knell, Thalwil, Switzerland [73] Assignee: Concast AG, Zurich, Switzerland [22] Filed: Oct. 16, 1969 [21] Appl. No.: 867,073

[52] US. Cl. ..164/83 [51] Int. Cl. ..B22d 11/02 [58] Field of Search ..164/83, 260, 261, 281, 71, 164/82 [56] References Cited UNITED STATES PATENTS 2,815,551 12/1957 Hessenberg et al ..164/83 3,025,579 3/1962 Littlewood ..164/154 3,118,195 l/l964 Gouzou et al. ..164/83 3,293,707 12/1966 Olsson ..164/83 FOREIGN PATENTS OR APPLICATIONS 1,266,961 9/1960 France 164/83 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Sandoe, Hopgood & Calimafde [5 7] ABSTRACT In continuously casting steel billets wherein the mold is oscillated up and down substantially according to a sine function the mean speed of descent of the mold is selected so that the maximum speed of withdrawal of the casting from the descending mold during not less than about 55 percent, and the minimum speed during not more than about 80 percent, of the time required for the complete descent of the mold is below the speed of the mold. Further, the level of liquid steel in the mold is preferably kept constant by adjusting the withdrawal speed of the casting in accordance with the rate of delivery of liquid steel into the mold; if the withdrawal speed of the casting exceeds the desired maximum or falls below the desired minimum, further delivery of liquid steel to the mold is stopped.

v 3 Claims, 1 Drawing Figure VELOCITY-TIME DIAGRAM FOR HARMONIC MOULD OSCILLATION DOWNSTROKE TIME 100% =|80 TIME OSCILLATION CYCLE ANGLE t RANGE l I STRANDS WITH OVERDRAWN OSCILLATORY MARKS.

RANGE 21 STRANDS WlTH GOOD QUALITY RANGE 5 I DANGER OF STICKING OF THE STRAND IN THE MOULD OSCILLATION DOWNSTROKE TIME 100% =I80 IK7IQ 5" 55% 99 .3 22.5" LLI I r 4o.5

o 0 270 RAINGE dMOULD VELOCITY -RANGE OF THE DESIRED STRAND RANGE 2 VELOCIT l h 5 RANGE 3 o id DOWNSTROKE UPSTROKE J oscILLATIoN CYCLE TIME ANGLE Ir PATENTED OCT 24. I972 VELOCITY-TIME DIAGRAM FOR HARMONIC MOULD RANGE l I STRANDS WITH OVERDRAWN OSCILLATORY MARKS.

RANGE 2: sTRANos WITH Gooo QUALITY RANGE 3 IDANGER OF STICKING OF THE STRAND IN THE MOULD R METHOD OF CONTINUOUSLY CASTING STEEL BILLE'IS The invention relates to a method of continuously casting steel billets which consists in pouring the steel into an open ended mold oscillating so that its speed changes substantially according to a sine function, and in withdrawing the casting at a speed governed by the existing casting conditions.

In continuous casting the liquid steel is poured into an open ended cooled mold of which the outlet end is initially closed by a dummy bar. The effect of cooling is to solidify the outer skin of the liquid steel inside the mold. The resultant casting which still has a liquid core is then withdrawn from the mold by drive means. Having left the mold the casting is conducted and. further cooled in a guideway.

A conventional method of preventing the casting from sticking to the inside wall of the mold is to provide relative motion between the casting and the mold. The withdrawal of a casting that sticks to the wall of the mold subjects the solidified skin to tensile stress which may open up cracks. As soon as such a weakened part of the casting skin ceases to be supported by the wall of the mold, it is unable to contain the ferrostatic pressure of the liquid core inside the casting and the liquid steel breaks out through the opening. When such a breakout of metal occurs the plant is liable to sustain major damage and the hazard to attendant personnel is also considerable.

In a prior publication the provision of continuous relative motion between casting and mold is considered to be a drawback because it impedes the transfer of heat from the casting to the mold wall and thus necessitates reducing the casting rate. It has therefore been proposed to generate a non-harmonic oscillatory motion of the mold in such manner that the downward speed of the mold corresponds to the speed of the casting and the ascent is very rapid. In practice a ratio of the times of downward and upward motions of 3 z 1 has proved to be advantageous. The motion of the mold is thus synchronized with the speed of the casting. Since relative motion between the mold and the casting is absent during the downward stroke of the mold, sticking cannot be entirely prevented. Consequently it has been proposed in another publication to provide slight relative motion between the mold and the casting during the downward stroke of the mold, the speed of the mold exceeding that of the casting by a few percent. This slight relative motion reduces the risk of sticking without significantly impairing the transfer of heat from the casting to the mold and, if the casting should nevertheless stick, it is detached from the mold whilst the solidified skin is being compressed. However, these non-harmonic oscillatory motions require considerable accelerational forces which generate vibrations that adversely affect the casting process. Hence the idea of non-harmonic oscillation has recently again been abandoned in practice.

It has also been proposed to oscillate the mold in a sinusoidal motion generated by an eccentric revolving in synchronism with the speed of rotation of the withdrawing unit. However, synchronization calls for the provision of expensive equipment, both when electrical or mechanical transmission means are employed.

It is the object of the presentinvention to prevent sticking of the casting to the inside wall of the mold and to prevent oscillation marks on the surface of the casting, which may lead to creasing when the castings'are rolled, by a simple and nevertheless operationally reliable method when the speed of the castingvaries during the casting process.

The drawing comprises a Velocity-Time diagram illustrating asine curve representing mold velocity during a single oscillation of a mold movedin simple harmonic motion, and indicating desirable and undesirable ranges of strand velocity.

According to the invention this is achieved by selecting a mean downward speed of the mold which is a constant value determined by existing castingconditions, and by keeping the maximum speed of the casting below the speed of the mold, during at Ieastabout 5 5 percent of the time required for the descent of the mold in each oscillatory cycle and by keeping the minimum speed of the casting below the speed of the mold during not more than about 80 percent of the time required for the descent of the mold in each oscillatory cycle.

According to a particularly. advantageous feature of the invention the level of the melt in the mold is kept constant by varying the withdrawal speed of the casting when the rate of delivery of the steel into the mold changes. If the withdrawal speed of the casting exceeds the desired maximum speed or falls below the desired minimum speed, further delivery of steel into the mold is stopped.

The method of the invention will be hereinafter described in greater detail.

The withdrawal speed of the casting is substantially a function of the casting conditions, such as the cross section of the casting, the quality of the steel, the temperature of the steel, the intensity of cooling, the occurrence of faults and so forth. Any change in the casting conditions during a pour or from one pour to the next may involve a change in the withdrawal speed of the casting. If the pouring nozzle is not controlled, the rate of delivery of the steel may change, for instance, as a result of this nozzle becoming gradually choked in the tundish or of the level of the steel in the tundish going down. In order to keep the surface of the melt in the mold at a constant level, any change in the rate of delivery of the steel into the mold is compensated by a corresponding change in the speed of withdrawal of the casting. Compensation is effected by hand or by conventional automatic systems.

Assuming that the cross section of the cast billet is X 70 mm, the speed of withdrawal under conventional casting conditions may be say 4.7 meters per minute. The mean speed of descent of the mold is preferably selected to be 30 percent to 40 percent higher than the speed of withdrawal of the casting, i.e. in the assumed case about 6.1 to 6.5 m/min.

The mean speed of descent of the mold is defined by:

where v mean speed in m/min n number of oscillations per minute h length of stroke in mm.

Assuming a frequency of n 200 and a mean mold speed of 6.5 m/min the length of the stroke according to this formula would be about 16 mm.

According to the present invention the maximum speed of withdrawal of the casting is to be below the speed of the mold during at least about 55 percent of the time required for the downward motion of the mold in each oscillatory cycle. The speed of the mold is determined by v v sina where is 180 when the duration of the descent of the mold is 100 percent and the maximum speed of the mold v exceeds the mean speed v,,, of the mold by 57 percent by virtue of the geometry of the sine curve.

For reasons of symmetry the angle for the determination of the maximum speed of the casting is 100 55) 2 22.5 percent of 180, i.e. 40.5. Consequently the maximum withdrawal speed of the casting will be 1.57 X 6.5 sin 40.5 E 6.7 m/rnin It was found that when this value is exceeded the risk of the casting sticking in the mold becomes unacceptably high.

The corresponding angle for the determination of the minimum speed of the casting that may occur is therefore (100 80) 2 10 percent of 180, i.e., 18. Consequently the minimum withdrawal speed of the casting will be 1.57 X 6.5 sin 18 5 3.2 rn/min.

Taking another example, a billet of carbon steel of 160 X 160 mm is cast at the rate of 1.4 m/min. At an average preadjusted constant downward speed of the mold of 2 m/min and a stroke of 10 mm the mold is oscillated at the rate of oscillations per minute. In such a case the speed of withdrawal of the casting must be kept between 1 and 2 m/min in order to obtain the desired advantageous results, i.e., the absence of overdrawn oscillatory marks and of any tendency of the casting to stick in the mold.

The idea which underlies the invention is also applicable to approximately sinusoidal mold motions generated, for instance, by conventional means, such as eccentrics, since the effects that arise are the same as those that have been described.

What is claimed is:

l. A method of continuous casting billets of steel,

com risin urin the steel into a coole en ende mol whic i oscfilated so that its spec rmg eac cycle varies substantially according to a sine function, maintaining the mean speed of descent of the mold at a predetermined constant rate, and withdrawing the casting at a variable speed governed by the existing casting conditions, characterized in that the maximum speed of withdrawal of the casting is lower than the instantaneous speed of descent of the mold which prevails during at least about 55 percent of the time required for a complete descent of the mold, and the minimum speed of withdrawal of the casting is higher than the instantaneous speed of descent of the mold which prevails during at least about 20 percent of the time required for a complete descent of the mold.

2. A method according to claim 1, which is characterized in that, whenever the rate of delivery of the steel into the mold changes, the level of the melt in the mold is kept constant by varying the speed of withdrawal of the casting, and stopping the further supply of steel to the mold when the speed of withdrawal exceeds the said maximum or falls below the said minimum value.

3. A method according to claim 1, characterized in that relative speeds of the descent of the mold and of the withdrawal of the casting are such that the mean speed of descent of the mold is from about 30% to about 40% higher than the speed of withdrawal of the castingv 

1. A method of continuous casting billets of steel, comprising pouring the steel into a cooled open ended mold which is oscillated so that its speed during each cycle varies substantially according to a sine function, maintaining the mean speed of descent of the mold at a predetermined constant rate, and withdrawing the casting at a variable speed governed by the existing casting conditions, characterized in that the maximum speed of withdrawal of the casting is lower than the instantaneous speed of descent of the mold which prevails during at least about 55 percent of the time required for a complete descent of the mold, and the minimum speed of withdrawal of the casting is higher than the instantaneous speed of descent of the mold which prevails during at least about 20 percent of the time required for a complete descent of the mold.
 2. A method according to claim 1, which is characterized in that, whenever the rate of delivery of the steel into the mold changes, the level of the melt in the mold is kept constant by varying the speed of withdrawal of the casting, and stopping the further supply of steel to the mold when the speed of withdrawal exceeds the said maximum or falls below the said minimum value.
 3. A method according to claim 1, characterized in that relative speeds of the descent of the mold and of the withdrawal of the casting are such that the mean speed of descent of the mold is from about 30% to about 40% higher than the speed of withdrawal of the casting. 